1. Field
The disclosed technology relates to systems for processing semiconductor substrates and, in particular, to systems for evaporating metal onto semiconductor substrates.
2. Description of the Related Art
Processing of semiconductor substrates, such as GaAs wafers, may include depositing a relatively thin film of metal onto the substrates by evaporation. Evaporation may be accomplished by heating a metal source and having the resulting metal vapor travel through a vacuum onto the substrate. The metal vapor may then condense on the substrate to produce a thin film of metal. The thin film may be used for a variety of purposes, such as serving as a contact for a collector, a base, and/or an emitter of a bipolar transistor.
While manufacturing wafers that will be used to make production parts, a test wafer may also be processed in order to make a quality assessment associated with the production parts and/or to ensure that processes, such as evaporation, were carried out within acceptable bounds. Small changes in environment may noticeably impact performance and/or reliability of end products produced from semiconductor wafers. Thus, the test wafer can be positioned in tools, such as an evaporator, so that the test wafer can be exposed to substantially the same processing environment as production wafers. As wafer diameter increases, fewer wafers can be positioned within the same amount of space. This leaves less space for test wafers, which may take away valuable space from production wafers.
While evaporation onto substrates takes place, sensors, such as crystal monitor sensors, may monitor conditions of evaporation. For example, a crystal monitoring system can detect a deposition rate and/or thickness of metal deposited onto the substrate. For accurate measurements, the crystal monitor sensor may be positioned near the production wafers so that the crystal monitor is exposed to substantially the same environment as the production wafers within an evaporation chamber. Information obtained using the crystal monitor sensor may be used to stop evaporation and prevent additional metal from being deposited onto the substrates.